A Boston police officer clears Boylston Street after explosions at the finish line of the 2013 Boston Marathon on Monday. / Charles Krupa, AP

by Kevin Johnson and Aamer Madhani, USA TODAY

by Kevin Johnson and Aamer Madhani, USA TODAY

Law enforcement authorities investigating simultaneous, deadly bombings that ripped through crowds at the Boston Marathon on Monday were pursuing potential witnesses across the region and reviewing streams of video gathered from nearby security cameras, as dozens of additional federal investigators were dispatched to Boston to assist in a broad terrorism inquiry.

WBZ-TV reported late Monday that law enforcement officers were searching an apartment in the Boston suburb of Revere. Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night in Revere but provided no further details.

Boston Police Commissioner Ed Davis said no suspects were in custody, contrary to various news reports indicating that at least one person had been detained. "We are questioning many people, but there is no suspect in custody," Davis said.

Two law enforcement officials said no other explosive devices had been found other than the two that detonated at the race scene. The officials said as many as seven suspicious packages were destroyed in controlled explosions, but they were later found not to be bombs.

The law enforcement officials, who have been briefed on the matter, were not authorized to comment publicly.

The two devices detonated on the marathon route were described by one of the officials as relatively small and possibly containing small ball bearings or BB gun pellets designed to serve as shrapnel.

It was unclear whether the devices were remotely detonated or included timers, the official said, adding that no conclusions had been drawn on whether an organized group or lone wolf had been responsible for the attack.

The two law enforcement officials said authorities were closely questioning a person who was seen fleeing from the scene and sought treatment at a hospital. The person, a Saudi national, is currently not considered a suspect, one of the officials said. That person is among hundreds being interviewed as part of the case.

Authorities also issued a bulletin for an unidentified man who was seen running from a restricted area. It was unclear, according to one of the officials, whether the person was anything other than a witness.

Late Monday, the FBI announced that it was taking over the case and that the Justice Department was sending contingents of additional FBI and Bureau of Alcohol Tobacco Firearms and Explosives agents to the city.

Richard DesLauriers, the FBI's top agent in Boston, said the federal government was bringing "substantial resources" to assist in the inquiry.

Davis said reported that three people were killed in the blasts. "This cowardly act will not be taken in stride," he said.

House Homeland Security Chairman Michael McCaul, R-Texas, said late Monday, "It has all the hallmarks of an act of terrorism. The White House is not calling it an act of terrorism yet. I am. We just don't know if it's foreign or domestic.

"One piece of evidence I'm particularly interested in is whether these devices contained ball bearings, because that's the signature of an IED," McCaul said. IEDs, or improvised explosive devices, are frequently used by al-Qaeda-linked groups and would suggest a foreign link, he said.

He noted that many similar planned attacks have been thwarted in recent years - the Times Square plot and the underwear bomber. "Unfortunately, this one was successful."

Davis said police were dispatched to hospitals, hotels and elsewhere to identify possible witnesses to the dual blasts that exploded 50 to 100 yards apart, near the iconic race's finish line. The commissioner initially said police were treating a third incident at the John F. Kennedy Library as related, but later a police spokeswoman said the library incident was fire-related.

Investigators cordoned off the chaotic marathon crime scene, which was littered with debris and abandoned backpacks. Davis said the packs were being examined for other possible explosive devices. "Anything that is unattended is being treated as a potential problem," Davis said. He said some were detonated in controlled blasts as a precaution.